The recovery of data from a transmitted signal containing digital video and related information at a digital receiver typically requires the implementation of three functions: timing recovery for symbol synchronization, carrier recovery (frequency demodulation) and equalization. Timing recovery is the process by which the receiver clock (timebase) is synchronized to the transmitter clock. This permits the received signal to be sampled at the optimum point in time to reduce the chance of a slicing error associated with decision-directed processing of received symbol values. In some receivers, the received signal is sampled at a multiple of the transmitter symbol rate. For example, some receivers sample the received signal at twice the transmitter symbol rate.
In any event, the sampling clock of the receiver must be synchronized to the symbol clock of the transmitter.
Carrier recovery is the process by which a received RF signal, after being frequency shifted to a lower intermediate frequency passband, is frequency shifted to baseband to permit recovery of the modulating baseband information. Equalization is a process which compensates for the effects of transmission channel disturbances upon the received signal. More specifically, equalization removes intersymbol interference (ISI) caused by transmission channel disturbances. ISI causes the value of a given symbol to be distorted by the values of preceding and following symbols. These and related functions are described in greater detail by Lee and Messerschmitt in Digital Communication (Kluwer Academic Press, Boston, Mass., U.S.A.).
Prior receivers required a relatively stable source of a sampling clock signal, yet controllable so that it could be locked to the transmitter symbol clock. Voltage controlled crystal oscillators (VCXOs) were used for this function. The clock signal produced by a VCXO is stable, but controllable over a relatively narrow range, so that it can be locked to the transmitter symbol clock. However, a voltage controlled oscillator such as a VCXO is an analog component, so it is relatively expensive, and prone to drift over its lifetime. In addition, if it is necessary to receive signals from different transmitters having different symbol clock frequencies (such as in European satellite systems), it is necessary to have a separate VCXO for each such transmitter, further increasing the cost of the receiver.
It is desirable to provide a symbol timing recovery system which can support more than one symbol rate. It is furthermore recognized as desirable to provide such a timing recovery system which offers performance benefits and cost benefits such as in terms of hardware requirements, compared to known types of timing recovery systems, for example those including multiple VCXOs for each received symbol rate.